The DOMA trap
I think they are walking into a trap in a more general sense, simply by re-affirming that this fight is so existentially important to them. Over the last few years, anti-gay prejudice has been losing ground in American society at a startling rate; from being the default viewpoint a decade or two ago, it's increasingly looking like an ugly eccentricity of cranky old blue-noses. A couple of polls have even shown support for gay marriage above 50%, and even if we're not actually there yet, it's clear we soon will be. Yet those cranky old blue-noses, a.k.a. the Christian Right, are the dominant force in right-wing politics these days; despite the efforts of secular conservatives like Mitch Daniels to shift the focus from "social" (theocratic) issues to economic ones, the Republican party cannot extricate itself from the weird obsessions which animate a key part of its base. And so it is tied to stances which increasingly alienate the mainstream, especially the young, who will not forget.
Let's hope that Obama's announcement also reminds our own side of what's at stake next year. Never forget that what happened in the 2010 election was not a swing to the right, but a collapse of voter turn-out -- too many people stayed home because they were disappointed in the Democrats or had fallen prey to the grotesque delusion that there are no real differences between Democrats and Republicans. As Obama's DOMA decision and the conservative reactions remind us, there are.